The fall webworm (Hyphantria cunea) is a moth in the family Arctiidae known principally for its larval stage, which creates the characteristic webbed nests on the tree limbs of a wide variety of hardwoods in the late summer and fall. It is mainly an aesthetic pest, and is not believed to harm otherwise healthy trees. It is well known to commercial tree services and arboriculturists.

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The moth is native to North America, ranging from Canada to Mexico, and is one of the few insect pests introduced from North America into other continents. Introduced to what was formerly Yugoslavia in the 1940s (firstly recorded in 1949[1]), it now has occupied probably its entire range in Europe from France to the Caspian Sea in the east, as well as penetrates into Central Asia: Turkmenistan (from 1990-1993), Uzbekistan (Fergana valley from 1996-1997), Kyrghyzstan, and southeastern Kazakhstan. It was also introduced into Japan in 1945, and has adjusted its number of generations per year since its arrival.[2] It spread into China, southern Mongolia, Korea and southern Primorsky Krai of Russia, so now is considered holarctic in distribution.

One generation per year emerges in the northern part of North America, with larvae appearing in late summer through early fall. South of an approximate latitude of 40°N there are two or more generations annually, with webs appearing progressively earlier further south.[3]

The caterpillars are highly variable in coloration, ranging from a pale yellow, to dark grey, with yellow spots and long and short bristles.[3] There are two cream stripes along the sides. The two races, one more common in the north, the other in the south, differ in head capsule coloration.[5] The maximum length is 35 mm. Webs are progressively enlarged, and much messier looking than those of tent caterpillars (which occur only in spring and have shorter hairs and very little yellow on their bodies) also (webs from the fall webworm are concentrated to the tips of the branches, where as the tent caterpillar webs are largely found in the unions). Larvae feed inside the tents until the late instars. Very young larvae feed only on the upper surfaces of leaves. Later, they consume whole leaves. The larval stage lasts about four to six weeks.[5]

The pupa stage overwinters in the bark and leaf litter at the base of the trees. It is dark brown and about 10 mm long. The thin brown cocoon is made of silk with bits of detritus interwoven.[5] This stage overwinters.

The adult is mostly white in the north, but in the south, it may be marked with black or brown spots on the forewings.[4][5] It is quite 'hairy', and the front legs have bright yellow or orange patches. The underwings will have less marking than the forewings, and the abdomen often has a sprinkling of brown hairs. It has a wingspan of 35–42 mm.

The fall webworm feeds on just about any type of deciduous tree, where leaves are chewed; branches or the entire tree may become defoliated. Worldwide, it has been recorded from 636 species,[6] and is considered to be among the most polyphagous of insects. In the eastern U.S., pecan, walnut, American elm, hickory, fruit trees, and some maples are preferred hosts; in some areas persimmon and sweetgum are also readily eaten. In the west, alder, willow, cottonwood and fruit trees are commonly used.[4]

In the video game Metal Gear Solid 2: Sons of Liberty, the AI Colonel, while infected by a computer virus, mentions that he was a North American Fall Webworm in his past life, reminiscing about how "those were the good old days."